The present invention relates to an electro-acoustic device or apparatus for the underwater signalling and identification of a vessel which is intended to prevent, during armed conflict, untimely underwater attack on neutral vessels, hospital ships, health-service vessels, rescue vessels and generally those craft protected by the Geneva Conventions.
The main object of the conventional submarine service is always that of destroying the merchant tonnage of the enemy or enemies so as to achieve their maritime blockade. The nature of anti-submarine warfare and the activities of underwater vessels require that these vessels remain deeply submerged, maintain silence on board and, by means of passive sonars, listen for underwater noises in their patrol zone. After a target has been detected, it is destroyed by different types of torpedo, for example of the cable-guided and acoustic types, which have ranges of up to twenty-five nautical miles. In these circumstances the detection and identification of a possible target are achieved solely by sound and practically never by sight; certainly, a submarine can hardly be allowed to surface with its periscope down because of the great danger of being detected and rapidly destroyed. When moving, a vessel emits an underwater noise e.g., the whirr of its propellers, the noise of the main and auxiliary engines, the wash along its stem and vibrations due to its advance through the water.
This noise forms the basis for detecting and identifying vessels by submarines with the aid of passive sonars. These latter are directional listening networks which cover the horizon sector by sector. They consist generally of hydrophones, compensation networks, amplifiers and various systems for processing signals. They are in the charge of trained operators who listen to the noises that are picked up and are aided by automatic indicators which have been developed largely to suit the various marine nations and the class of submarine. As a rule, listening by trained operators is the simplest method and is in general use in all types of submarines.
Increase in the noise level at the outputs of the passive sonar at a given bearing leads to the detection of a possible target. The spectral characteristics of the signal (acoustic signature) permit a friend or an enemy to be detected and enable the type of craft to be identified (cargo vessels, submarine chasers, fighting vessels, etc.).
Although, in theory, it is possible to identify any vessel by its acoustic signature, in practice this can only be achieved in the case of certain known vessels or types of vessels of each maritime nation.
In particular, it is practically impossible to differentiate between, on the one hand, neutral transport vessels and those protected by the Geneva Conventions, and, on the other hand, transport vessels belonging to the enemy. In the event of war, the first-mentioned vessels will be regarded as non-friendly and will therefore run a very grave risk of being sunk without warning. In these circumstances, a naval power will be able to exercise a total maritime blockade against neutrals and the hospital services by claiming that it is impossible to guarantee immunity to their vessels in view of the submarine danger.
Such a situation is grave for neutrals, and in particular for Switzerland, whose supplies by sea route would be dangerously threatened or even cut off in the event of a crisis or armed conflict. Furthermore, because of this bias, neutrals would be subjected to intolerable political pressures by the belligerents.
Hitherto, no underwater acoustic signalling and identification system has been developed and used. It will be remembered, however, that in 1917, an agreement between the belligerent nations had provided that allied hospital ships should be accompanied by paddle vessels. Their particular acoustic signature enabled hospital ship convoys to be identified by German submarines. In 1972, at the Conference of government experts regarding the reaffirmation and development of international human rights regarding armed conflicts, the International Red Cross Committee dealt with the signalling and identification of hospital ships. The problem of protection against underwater attacks was raised in the technical memorandum (Geneva, April 1972). It was there suggested that an audible underwater signal having a suitable pitch should be sent out. Australian experts proposed the introduction into the technical appendix of the Geneva Conventions, a new clause relating to underwater acoustic signalling of the presence of hospital ships. The context of this clause is as follows:
1. Ships and places of embarkation concerned with hospital services could be identified by a signal transmitted by a multi-directional sonar. PA0 2. The strength of the emitted signals was to be at least equal to that radiated in the water in the emission frequency band by the vessels in question at their maximum cruising speed. PA0 3. The signal was to be transmitted sequentially at frequencies of 3, 6 and 12 kHz and was to consist of the letters HS (Hotel Sierra) in morse code and transmitted three times at each frequency, each group of three transmissions being followed by a continuous signal of five seconds at the same frequency. The transmission cycle would be repeated at intervals of two minutes maximum.
This proposal has not yet resulted in the setting up of a signalling system, and on the basis of calculations carried out by the present inventors, this system would not be able to operate satisfactorily.